Looks to me as if you need one measurement and a subtraction.
I'd say you just have to get a reading from your killawatt with 3 cards out of the 6. Subtract that value from 1210 W to get XXX. Then your total power consumption with 6 more cards, dual mining and your BIOS will be 1210 W + 2*XXX.

But do flash your new cards with your own BIOS before you plug them all.